The ametropic human eye has refractive errors that in first approximation can be described in terms of a sphere, a cylinder, and an axis orientation. This is based on the assumption that a visual defect can be approximately corrected through a lens with simple surfaces such as toroids and spheres. This approximation may correct an error in the refraction of light rays that enter the center of the eye pupil.
While it is customary to determine the refractive errors of the human eye by relying on the subjective refraction of the patient under examination when presenting to him a plurality of optotypes through lenses of different refractive power, so-called subjective refraction or manifest refraction, the possibility of measuring the refractive errors of the eye has now been available for several years with the so-called objective refraction. Moreover, it is possible to measure the refractive power of the eye over the entire pupil. The measurable errors include for example spherical aberration, coma, trefoil error, higher orders of spherical aberration, etc. In certain implementations, the objective refraction method is based on determining the wavefront of a propagating light bundle. The functional principal of a wavefront refractor is described in document U.S. Pat. No. 6,382,795 B1, and also includes a synopsis of a plurality of different variants.
The refractive errors or imaging errors of the human eye can be mathematically described by means of so-called Zernike polynomials. The errors of the eye in regard to sphere, cylinder, and axis can be described, for example, through second-order Zernike polynomials. These errors are therefore often referred to as second-order aberrations or lower order aberrations. Further errors can be described through higher-order Zernike polynomials. Therefore, these errors are in general referred to as higher-order aberrations. The information gained from a wavefront refractor can be used in the development of improved vision aids or improved eyesight correction methods. A well-known example for an eyesight correction method is the procedure of wavefront-guided refractive surgery. In this procedure, a volume of any desired geometry is removed from the surface of the cornea in order to correct refractive errors, including those of a higher order. In general, in order to determine an eyeglass prescription for visual aids, an eye care professional determines several parameters. In the case of spectacle lenses, for example, the most relevant ones are: refractive values, usually given in form of sphere, cylinder, and axis; fitting parameters, such as pupil distance, fitting height, pantoscopic angle, and others; and near vision addition, for example, in case of progressive lenses. For contact lenses, the set of parameters usually includes at least the refractive values, similar to spectacle lenses, and corneal curvature.
A basic criterion for objective refraction algorithms was suggested to be that the objective refraction that most closely matches the subjective refraction data is considered best. This was for example suggested in document U.S. Pat. No. 7,857,451 B2. Document U.S. Pat. No. 7,857,451 B2 shows a method and system for determining the appropriate refraction prescription in the clinical optometry or ophthalmology setting. Data in the form of aberrometric input, patient history and other information, and/or other environmental data are used to optimize a real-world prescription for an individual's optic needs through the use of an equivalent quadratic fitting calculation or a simulated through-focus experiment. A corresponding disclosure is made in document WO 2013/058725 A1 of the same patent family.
Wavefront aberration data is used to obtain the objective estimates of optimal second order corrections for wearers. These prescriptions obtained by objective refraction can sometimes vary significantly from the same wearer's prescription obtained via subjective refraction. This may be disadvantageous if the previous or new subjective prescription is judged superior.
Document US 2005/0057723 A1 shows method of measuring eye refraction to achieve desired quality according to a selected vision characteristics comprising the steps of selecting a characteristic of vision to correlate to the desired quality of vision from a group of vision characteristics comprising acuity, Strehl ratio, contrast sensitivity, night vision, day vision, and depth of focus, dynamic refraction over a period of time during focus accommodation, and dynamic refraction over a period of time during pupil constriction and dilation; using wavefront aberration measurements to objectively measure the state of the eye refraction that defines the desired vision characteristic; and expressing the measured state of refraction with a mathematical function to enable correction of the pre-selected vision characteristic to achieve the desired quality of vision. The mathematical function of expression may be a Zernike polynomial having both second order and higher order terms or a function determined by spline mathematical calculations. The pre-selected desired vision characteristics may be determined using ray tracing technology.
There remains a need in the art to determine an eyeglass prescription for an eye that only has a small optical difference to the prescription obtained via subjective refraction.